The present invention relates to a method of operating an internal combustion engine in a motor vehicle in particular where air is drawn in through a throttle device, through a throttle valve, into a combustion chamber in particular, where fuel is injected directly into the combustion chamber during an intake phase in a first mode of operation or during a compression phase in a second mode of operation and ignited there, and a fuel-air mixture is drawn into the combustion chamber through a fuel tank vent. The present invention also relates to a control unit for an internal combustion engine of a motor vehicle in particular and an internal combustion engine for a motor vehicle in particular.
Prior methods, control units, and internal combustion engines are known with direct gasoline injection, for example, where fuel is injected into the combustion chamber of the internal combustion engine during the intake phase in homogeneous operation or during the compression phase in stratified charge operation. Homogeneous operation is preferably provided for full-load operation of the internal combustion engine, while stratified charge operation is suitable for idling and partial load operations. Such a direct injection internal combustion engine switches between these modes of operation as a function of the required torque, for example.
In homogeneous operation, a fuel-air mixture can be supplied to the combustion chamber through a fuel tank vent. Since the air taken in through the throttle valve and the fuel mass injected directly into the combustion chamber during the intake phase are swirled homogeneously until ignition, this results in a homogeneous mixture in the combustion chamber on the whole. The lambda value of the fuel-air mixture in the combustion chamber may thus be controlled and/or regulated at a setpoint by a lambda regulation unit.
In this way, in homogeneous operation an activated carbon filter that has become loaded with fuel can be unloaded again through such a fuel tank vent. Fuel evaporating in a fuel tank can thus be utilized through the fuel tank vent. However, such unloading requires a relatively long period of time. As a result, the internal combustion engine may be operated in fuel-saving stratified charge operation only briefly.
In stratified charge operation, there is the risk that fuel-air mixture additionally drawn in through the throttle valve and the fuel tank vent might not be ignited because of its low fuel content, and thus unburned fuel would be discharged into the environment.
An object of the present invention is to provide a method of operating an internal combustion engine with which the lowest possible fuel consumption is possible while at the same time achieving the lowest possible emission of unburned fuel into the environment.
This object is achieved with a method according to the present invention by drawing a lean fuel-air mixture into the combustion chamber through the throttle valve and through the fuel tank vent in the second mode of operation, and igniting the lean fuel-air mixture by additional fuel injected and ignited in the combustion chamber. This object is achieved accordingly with a control unit and an internal combustion engine according to the present invention.
The additional fuel injected and ignited in stratified charge operation results in so-called flare ignition, in which the lean fuel-air mixture drawn in through the throttle valve and through the fuel tank vent valve is ignited.
The fuel-air mixture drawn in through the throttle valve and the fuel tank vent should be as lean as possible. It need not necessarily be ignitable by a spark. At the same time, however, the fuel-air mixture should be rich enough so that when ignited by the flare, it will burn completely. Therefore, the throttle valve and the fuel tank vent valve are adjusted accordingly.
The degree of loading of the activated carbon filter is determined by brief trial venting and through the effects thereof on the lambda value of the exhaust gas. These trial ventings should take place during homogeneous operation if possible because the lambda value of the exhaust gas can then be determined with the greatest accuracy.
With a view to the efficiency of the internal combustion engine, the lowest possible throttling is set. The fuel tank vent valve and the activated carbon filter may optionally be designed with larger dimensions than usual to allow them to deliver the required amounts of fuel. For the same reasons, venting is appropriate only beyond a certain filling level of the activated carbon filter.
Thus, on the whole this method makes it possible to use fuel tank venting not only in homogeneous operation but also in stratified charge operation. Complete combustion of the air-fuel mixture supplied through the fuel tank vent is guaranteed through the additional fuel mass which is injected directly and ignited. Thus, there is no unburned fuel to pollute the environment.
At the same time, the efficiency of the internal combustion engine in stratified charge operation is improved through the air-fuel mixture supplied additionally through the fuel tank vent. Stratified charge operation may be used to produce a greater torque due to the air-fuel mixture additionally drawn in through the fuel tank vent. Thus, it is less frequently necessary to switch to homogeneous operation, which consumes more fuel.
Likewise, operation of the fuel tank vent also in stratified charge operation yields the result that the entire fuel tank venting system becomes more efficient, and more evaporated fuel can be supplied for combustion than in the past. This represents an additional saving of fuel and at the same time a reduction in pollutants produced.
It is self-evident that the use of the fuel tank vent according to the present invention in stratified charge operation can be combined with the known use of fuel tank venting in homogeneous operation.
In an advantageous embodiment of the present invention, the total fuel mass injected is composed of the mass of the fuel contained in the lean air-fuel mixture drawn in through the fuel tank vent and the mass of the additionally injected fuel.
It is especially advantageous if the mass of the additionally injected fuel is determined as a function of the total fuel mass injected and of the mass of the fuel contained in the lean air-fuel mixture drawn in through the fuel tank vent.
This guarantees that the torque generated by the internal combustion engine can be controlled and/or regulated exactly at the desired torque by the control unit. The control unit here takes into account first the torque resulting from the air-fuel mixture as well as the torque resulting from the directly injected fuel mass.
In an advantageous refinement of the present invention, no additional fuel is injected if the lean air-fuel mixture alone is ignitable. This represents homogeneous lean operation of the internal combustion engine, which is achieved only through fuel tank venting.
Although the efficiency of the internal combustion engine is not as good in this case as that in stratified charge operation, it is possible in homogeneous lean operation to regenerate the activated carbon filter without switching to lambda-equals-one operation, which is even worse from the standpoint of efficiency. This is especially advantageous if, for example, the engine is operated for a long period of time at very low loads and high outside temperatures and if the activated carbon filter threatens to overflow, because in this case in stratified charge operation without throttling no fuel tank venting is possible because of the torque produced additionally in that case.
Another possibility is to open the throttle valve and the fuel tank vent valve only partially in stratified operation with flare ignition and thus to have a transition to throttled operation.
However, both of the possibilities described here are inferior to unthrottled stratified charge operation, so they should be used only in the exceptional situation described above.
It is especially advantageous here if the fuel tank vent and/or throttle valve is throttled to influence the resulting torque. This prevents an increase in the torque generated by the internal combustion engine above the torque desired by the driver.
Another possibility of rapidly regenerating the activated carbon filter is obtained in homogeneous operation during the unloading phases of an NOx storage catalyst. During these unloading phases, operation should be at lambda equals one. The mixture can then be made richer with the help of fuel of the fuel tank venting. The air-fuel mixture produced additionally through the fuel injector may even be lean.
Implementation of the method according to the present invention in the form of a control element provided for a control unit of an internal combustion engine in a motor vehicle in particular is especially important. A program capable of running on a computer, in particular a microprocessor, and suitable for execution of the method according to the present invention is stored in the control element. Thus, in this case the present invention is implemented by a program stored in the control element, so that this control element provided with the program represents the present invention in the same manner as the method for whose execution the program is suitable. In particular, an electric storage medium may be used as the control unit, e.g., a read-only memory or a flash memory.
Additional features, possible applications and advantages of the present invention are derived from the following description of embodiments of the present invention as illustrated in the drawing. All the features presented or described here, either alone or in any desired combination, form the object of the present invention, regardless of how they are formulated or presented in the description and/or drawing.